47kW

47kW

10kW

Microphone,

Hydrophone

etc.

Thermocouple

Center-tap provides

bias current return.

INA129

INA129

INA129

INA128-HT, INA129-HT

SBOS501F – JANUARY 2010 – REVISED FEBRUARY 2015

www.ti.com

Typical Application (continued)

9.2.2.4 Input Bias Current Return Path

The input impedances of the INA128-HT and INA129-HT are extremely high (approximately 1010

Ω). However, a

path must be provided for the input bias current of both inputs. This input bias current is approximately ±50 nA.

High input impedance means that this input bias current changes very little with varying input voltage.

Input circuitry must provide a path for this input bias current for proper operation. Figure 22 shows various

provisions for an input bias current path. Without a bias current path, the inputs will float to a potential which

exceeds the common-mode range, and the input amplifiers will saturate.

If the differential source resistance is low, the bias current return path can be connected to one input (see the

thermocouple example in Figure 22). With higher source impedance, using two equal resistors provides a

balanced input with possible advantages of lower input offset voltage due to bias current and better high-

frequency common-mode rejection.

Figure 22. Providing an Input Common-Mode Current Path

18

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